1. Field of the Invention
The present invention relates generally to current measuring devices, and more particularly to a system and method for automatically processing and calibrating data from a current measuring device.
2. Related Art
For several decades, scientists have been measuring the direction and velocity of water currents for a variety of reasons. For example, oceanographers and seismologists often profile ocean currents at various depths for scientific study. In addition, ocean currents are often measured to provide information to support offshore drilling operations and the like. Currents have also been measured in smaller bodies of water, such as rivers, for example, to compute flow and river discharge.
Typically, currents are measured from moving vessels ranging in size from small rafts to large ocean-going ships. Current measuring devices (also referred to herein as xe2x80x9ccurrent sensorsxe2x80x9d and xe2x80x9ctransducersxe2x80x9d) are typically mounted in the midsection of a ship""s hull. The current sensors measure the direction and velocity of currents at one or more predefined depths at repeating intervals of time.
An example of a presently used type of current sensor is an acoustic Doppler current profiler (xe2x80x9cADCPxe2x80x9d or xe2x80x9cADPxe2x80x9d) device. Examples of such devices are the VM-ADP, manufactured by Nortek AS of Oslo, Norway, and the VM-ADCP, manufactured by RD Instruments of San Diego, Calif. These current sensors transmit and receive sound waves and calculate current velocity and direction based on the Doppler shift of the transmitted sound waves.
The current sensors themselves often provide all of the information necessary to compute the direction and velocity of currents to the satisfaction of the user. However, many users require more precise data and must rely on systems that incorporate data from external navigational devices such as gyrocompasses, GPS receivers and the like.
These external devices are used to calibrate the system so that current measurements can be independent of the ship""s hull, velocity and the orientation of the transducer itself. Thus, calibration is a very important issue that controls not only the quality of the data being collected, but also the usability of the entire system. Accordingly, a calibration coefficient must be calculated to account for the components in the current measurement dataset that are caused by the orientation offset of the transducer, the ship""s rotation, and the currents interfering with the ship""s hull.
Generally currents are measured as vectors relative to the vessel""s frame of reference. However, users generally need the current flow data relative to the fixed earth frame of reference. Thus, the collected data must be transformed to the reference frame of the earth. This transformation generally has three parts: the rotation of the measured current velocity to a direction relative to true north; a scale factor correction of the measured velocity; and the removal of the velocity of the vessel relative to the earth.
Well-known calibration algorithms and techniques are in general use today as evidenced by the following three articles published in the Journal of Atmospheric and Oceanic Technology (by the America Meteorological Society): Terrence M. Joyce, On In Situ xe2x80x9cCalibration of Shipboard ADCPs (1989); Gwyn Gfiffiths, Using 3D GPS Heading for Improving Underway ADCP data (1994); and C. N. Flagg, et al, Operating an Acoustic Doppler Current Profiler aboard a Container Vessel (1998). The preceding three articles are hereby incorporated herein by reference.
The problem is that calibrating present current measuring systems require an enormous amount of effort and expert knowledge. For example, in the above referenced article by Flagg et al. it is stated that: xe2x80x9c[T]he calibration of the ADCP""s heading offset and velocity scaling is very time consuming, but fortunately only needed to be performed when the transducer was reinstalled during dry-docking and when the AGPS was installed.xe2x80x9d In the preceding quotation, the AGPS is a presently used navigation system.
Using present systems, many manual steps are required to successfully calibrate a vessel-mounted current measurement system. These steps include navigating the vessel through a predefined course, manually examining the collected data, determining an appropriate algorithm, averaging the data, manually calculating the calibration coefficient by solving one or more equations, and manually entering the calibration coefficient into a computer program. These manual steps not only allow for the introduction of many errors, but also generally require advanced skills and expert knowledge. More specifically, the steps required by present systems in use today can be generalized by the following seven steps listed below.
1. Locate and move the ship to an appropriate site for calibration. Calibration sites are chosen for the presence of weak or steady currents or for the ability to effectively bottom- track. Steady currents are required for the success of some calibration methods. Bottom-tracking refers to a well-known method where current sensors are used to directly measure the vessel""s velocity over the sea bed. Bottom-tracking is not possible unless the water depth is shallow enough so that the transducer can track the sea bed. In addition, bottom tracking is prone to errors in smaller bodies of waters where moving sediment at the bottom can introduce errors.
2. Move the ship through an appropriate course within the site. A typical pattern is one in which the ship moves over a course, then quickly reverses and covers the same course heading the opposite direction. Once this is performed, the user generally must assess whether the course was adequate for the calibration.
3. Manually identify the course segments (i.e. start/stop times) to be used for the calibration computations. Typically, this involves keeping a steady course and coordinating latitude and longitude data with physical observations made by the user to identify a proper course segment.
4. Screen the data for quality: reject bad data points and assess whether the overall data quality is adequate for calibration.
5. Compute average velocities from each segment, where each raw velocity estimate is rotated by the ship""s heading.
6. Solve one or more equations to obtain the one or more calibration coefficients.
7. Enter the calibration coefficients into real-time and/or post-processing software, keeping track of signs, offsets and scale factors.
As can be imagined, these steps have proven to be not only burdensome and difficult, but time consuming and prone to errors. In addition, users having a high degree of skill and knowledge are generally required to perform the above steps. Sometimes it is difficult or impossible to take make calibration runs, as indicated by step 2, above. For example, it is difficult, costly, and time consuming for seismic survey ships to turn around and repeat a course for the purposes of making a calibration run.
Further, due to the difficulty in performing these steps using the conventional methods outlined above, the steps are generally performed only once, and some steps may be skipped altogether. This can lead to erroneous data.
Therefore, what is needed is a system and method facilitating the calibration of oceanographic current measuring systems.
Accordingly, the present invention is directed toward a system and method for automatically calibrating an oceanographic current measuring device, that requires limited and non-expert human intervention. One feature of the present invention is that a system can be calibrated without requiring users to perform specific calibration runs. Another feature of the present invention is that multiple realizations of calibrations can be effortlessly performed.
The automated system and method of the present invention includes a means for automatically determining viable data segments that can be used for calibration purposes. A raw current dataset is defined herein as a current dataset comprising uncalibrated current data as measured by the current sensor.
During or after the construction of a raw current dataset, the present invention processes the data to determine the existence of one or more calibration opportunities. This is accomplished by searching the dataset for particular patterns in accordance with one or more particular calibration methods. Once a calibration opportunity is found, the present invention calculates a calibration coefficient therefrom. In addition, a preferred embodiment of the present invention calculates a quality factor associated with each calibration coefficient.
The quality factor is based on operating conditions and other factors that relate to the validity of the assumptions made in calculating the associated calibration coefficient. User input can also be used in determining a quality factor. In one embodiment, a user database is constructed based on answers to interrogatories presented in the form of an on-line questionnaire. This database is used to determine available calibration methods for a particular survey and may be used to influence the selection of a particular calibration coefficient.
Calibration coefficients and their associated quality factors are stored in a calibration realization table. A selection algorithm is used to select the best calibration or most optimal coefficient from the realization table. A calibrated current dataset is constructed from the selected calibration coefficient. Users can also specify which calibration coefficient to be used. The present invention assists the user in this task by presenting statistical information and quality data related to the multiple realizations of the calibration coefficients.
Users can also specify particular start and stop data points of a particular data segment that is to be used for calculating a coefficient. For example, in one embodiment, the user specifies to the system that a calibration run is about to begin. The user can also specify the end of the calibration run. In addition, the user can, at any time, force the system to calculate a calibration coefficient based on a specified data segment.
Further, users can change parameters used to identify the calibration opportunities. In one embodiment, the present invention automatically searches back through the raw current dataset to re-evaluate calibration opportunities, whenever parameters are changed by the user.
In another embodiment the present invention performs post processing on a raw current dataset. That is, in one embodiment, the present invention is used to post-process data previously collected in order to find the best possible calibration coefficient. The calibration coefficient is then used to re-create a calibrated current dataset. In this fashion, any previously collected dataset, including those in historical archives, can be used with the present invention so that more accurate results are obtained therefrom.
In one embodiment, the present invention assists the user in making calibration runs by dynamically displaying the vessel""s path in real-time. The visual display typically includes an optimal track to assist in the navigation process.
An advantage of the present invention is that it continuously evaluates the raw current dataset real-time to automatically determine the best calibration method.
Another advantage of the present invention is that it continually assesses the quality of the data and updates the calibration coefficient whenever necessary.
Another advantage of the present invention is that a system can be calibrated without requiring users to perform specific calibration runs.
Another advantage of the present invention is that multiple realizations of calibrations can be effortlessly performed.
Another advantage of the present invention is that is provides continual, automatic identification of suitable data for calibration during the normal course of data collection.
Another advantage of the present invention is that is provides guidance through standard calibration procedures and provides feedback to users on a real-time basis as to the quality of the calibration run.
Another advantage of the present invention is that is provides automated calibration using all available calibration methods, combined with continual quality assessment and evaluation so that up-to-date optimum of calibration is achieved.
Another advantage of the present invention is that is provides continual improvement of the calibration quality as new data suitable for calibration is obtained.
Another advantage of the present invention is that is provides automatic recalculation of previously collected data as the calibration changes and improves.
Another advantage of the present invention is that is provides upon-demand notification of the present quality of calibration in terms of the estimated uncertainty of measured data.
Another advantage of the present invention is that is provides visual feedback to allow users to assess the data used to form calibrations and to enable them to influence the process.
Another advantage of the present invention is that is provides highly knowledgeable users with access to recorded data in appropriate formats to allow them to independently reproduce calibration computations and verify the calibrations to their satisfaction.
Another advantage of the present invention is that it can be used by users having no previous expert knowledge or experience.
Another advantage of the present invention is that is provides opportunities for users to provide input to the calibration process on a sliding scale in accordance with their desires and abilities.